by arledgsc Wed Sep 11, 2013 9:49 am
A bias voltage issue should one of the easiest problems to isolate. Other than the VTA board exchange I am assuming that no other wiring in amp changed and use to work OK with the older board. First double check all the VTA board part installations (electrolytic cap orientation, resistor values/locations, and VTA board properly screwed down to the chassis making the ground connection), wiring, and solder quality. Remove all tubes and rectifier tube or sub, set the four bias voltage adjustment pots to mid-range, turn the amp on, and check the bias voltage feeds (all negative DC values below ground potential). Since you have two identical circuits (left and right) you can generally go back and forth at similar circuit stages for comparison (since left rear seems to work somewhat). Start at the diode rectifier and work your way through bias voltage circuits comparing left and right readings as they should be very close. When the voltages differ concentrate on that part of the circuit to understand why. You have no high voltage or tubes installed using this basic debug method so if the bias voltages look OK at this point you may have a bad component like a coupling capacitor... But first things first - do the basics and work your way up....
I just thought of something else to check first. When mounting the VTA board you did place nut spacers underneath the board? This adds needed clearance between the VTA board and chassis or otherwise you could short out the bias voltages and audio signal before they get to the output tubes. With no power applied check continuity (ohmage) of the wire feeding output to grid from the VTA board (output tube pin 5 to ground). Normally they should read each about 50K ohms with the bias pots set mid-range. WARNING: No power is applied and the high voltage capacitors allowed to bleed before attempting.